Merge bitcoin/bitcoin#22550: test: improve `test_signing_with_{csv,cltv}` subtests (speed, prevent timeout)

12f094ec215aacf30e4e380c0399f80d4e45c345 test: use constants for CSV/CLTV activation heights in rpc_signrawtransaction (Sebastian Falbesoner)
746f203f1950a7df50b9a7de87a361cc7354ffb4 test: introduce `generate_to_height` helper, use in rpc_signrawtransaction (Sebastian Falbesoner)
e3237b1cd07a5099fbb0108218194eb653b6a9f3 test: check that CSV/CLTV are active in rpc_signrawtransaction (Sebastian Falbesoner)

Pull request description:

  This PR primarily aims to solve the current RPC timeout problem for test rpc_signrawtransaction.py, as described in #22542. In the course of that the test is also improved in other ways (see https://github.com/bitcoin/bitcoin/pull/22542#pullrequestreview-714297804).

  Reviewers guideline:
  * In `test_signing_with_cltv()`, a comment is fixed -- it wrongly referred to CSV, it should be CLTV.
  * As preparation, assertions are added that ensure that CSV and CLTV have been really activated after generating blocks by checking the 'softforks' output of the getblockchaininfo() RPC. Right now in master, one could remove (or decrease, like in #22542) the generate calls and the test would still pass, when it shouldn't.
  * A helper `generate_to_height()` is introduced which improves the previous way of reaching a block height in two ways:
      - instead of blindly generating TH blocks to reach target block height >= TH, the current block height CH is taken into account, and only (TH - CH) are generated in total
      - to avoid potential RPC timeouts, the block generation is split up into multiple generatetoaddress RPC calls ([as suggested by laanwj](https://github.com/bitcoin/bitcoin/pull/22542#issuecomment-886237866)); here chunks of 200 blocks have been chosen
   * The helper is used in the affected sub-tests, which should both speed-up the test (from ~18s to ~12s on my machine) and avoid potential timeouts
   * Finally, the activation constants for CSV and CLTV are used instead of using magic numbers 500 and 1500

  Open questions:
  * Any good naming alternatives for `generate_to_height()`? Not really happy with the name, happy to hear suggestions
  * Where to put the CSV and CLTV activation height constants in the test_framewor folder? I guess importing constants from other tests isn't really the desired way to go

ACKs for top commit:
  laanwj:
    Code review and tested ACK 12f094ec215aacf30e4e380c0399f80d4e45c345
  rajarshimaitra:
    reACK https://github.com/bitcoin/bitcoin/pull/22550/commits/12f094ec215aacf30e4e380c0399f80d4e45c345

Tree-SHA512: 14509f6d3e5a5a05d6a30a3145bb82cd96a29d9d8a589abf1944a8bf34291cde78ce711195f52e9426588dc822b3618ec9b455e057360021ae46152bb7613516

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